The present invention is concerned with the fast control of current in inductive electrical loads, such as solenoids, particularly but not exclusively in automotive electronic control systems.
Inductive loads, such as solenoid coils, are typically controlled by means of a switch, such as a switching transistor, connected in series with the load across a voltage supply. In automotive applications, one side of the load (referred to as the “low side”) is normally connected to ground/chassis and the other side (referred to as the “high side”) is coupled to the non-grounded side of the voltage supply. For the purpose of monitoring/measuring the current through the load, a sensing element such as a resister is placed in series with the load and the voltage drop across this resistor is measured.
Traditional technology often used current sensing near the load driving transistor, such that current monitoring was only available when the drive was turned on. When the level of the monitored current was to be used for control of the switching transistor, this arrangement therefore had poor control.
Some known arrangements have used high side control of the load using P channel MOSFET devices, but these are relatively expensive.
As is well known, the current in an inductive load decays with time when the voltage supply is removed and special circuitry must be provided to dispose of this current. The conventional practice is to achieve this by the provision of a recirculating diode disposed in parallel with the load which turns on automatically to provide a current path back to the supply. However, the rate at which a diode disposed across the load in this manner can dissipate the recirculating current is relatively poor and the current in the load therefore falls off only slowly (see curve X in FIG. 3 of the attached drawings).
Known means for achieving faster control of the current turn-off in inductive loads have typically used two MOSFET devices per channel, which has an attendant cost.